This invention relates to a welding wire for automatic arc welding, the welding wire being of the class containing a relatively large amount of nickel and constituted of a tubular metal casing and a core composition of alloying elements with which the interior of the casing is filled. This welding wire is suitable for use in build-up welding for the purpose of providing hard surface to a metal body such as a metal die made of cast iron.
In the production of automobiles as a typical example of mass production of mechanical articles, press-forming of sheet metal plays an important role in producing various parts of the car bodies. Press-forming is a type of plastic working wherein the workpiece is subjected to relatively great contact pressure but undergoes deformation at relatively low speed. Despite the relatively low speed of deformation or sliding of the workpiece along the die face of the metal press-forming die, abrasion or wear of the die face is a matter of important concern in the case of mass production. According to the recent trend in the design of car bodies, there is an increasing need of performing very deep drawing or very severe and precise bending or stretching to obtain desirably shaped car body parts such as panels. Therefore, greater attention is paid to the wear resistance and antifrictional property of the metal die for such press-forming operations. As will be understood, wear of the die face results in lowering in the dimensional precision of the shaped article and raises the need for considerable time and labor to repair the die and remedy the unsatisfactory product. Where the antifrictional property of the die face is insufficient, the press-forming operation is liable to suffer from seizing between the die face and the workpiece brought into sliding contact with the die face and results in the appearance of scratches on the surface of the shaped article, and/or undesirable in the surface roughness of the shaped article. Currently prevalent to use cast iron as the material of relatively large-sized metal dies for press-forming of steel sheet, and of course there is the need of providing a hard surface to such metal dies by certain kinds of surface treatment.
Hard chromium plating is an example of surface treatment techniques to provide cast iron bodies with a hard, wear-resistant and fairly antifrictional coating layer. However, in the case of large-sized metal dies for deep drawing or severe stretch forming, the plated chromium layer is liable to peel off the cast iron surface during repeated press-forming operations. Therefore, it has been the practice to form a hard coating layer by an overlaying welding or build-up welding process. However, hitherto developed iron base welding materials for this purpose are generally unsatisfactory primarily because of the high probability of the occurrence of weld cracks. Besides, the deposited metal resulting from any of such welding materials is insufficient in its antifrictional property, so that the use of a metal die treated by this welding method tends to result in considerable scratching of the shaped articles. Furthermore, the coating on the die face itself undergoes considerable wear. Therefore, in this case it becomes necessary to use a highly effective lubricant in the press-forming process.
Recently developed nickel base and iron-nickel base welding materials are said to be suitable for use in build-up welding operations for the purpose of overlaying cast iron bodies with a hard and antifrictional coating layer. It is true that when these welding materials are applied to metal dies the probability of scratching of the shaped articles can be greatly reduced even when press-forming is performed with insufficient lubrication. However, these nickel or iron-nickel base welding materials are generally insufficient in the wear resistance of the deposited metal, so that the metal dies treated with these welding materials are short in service life and are liable to cause lowering of the dimensional precision of the shaped articles and, particularly in the case of shaping relatively large-sized panels, wrinkling of the shaped panels.
These nickel or iron-nickel base welding materials are mostly in the form of covered electrodes for use in shielded metal arc welding processes. Compared with an automatic arc welding process using a continuous welding wire, a shielded metal arc welding process using a covered electrode requires considerable manhours for replacement of the electrodes, removal of slag from the surface of the deposited metal and subsequent brushing of the weld zones. In the case of a build-up welding operation it is usual to perform multi-layer welding, and, therefore, the large number of manhours necessary for such auxiliary procedures in the use of such covered electrodes becomes a matter of great concern. There is a keen demand for a welding wire for use in automatic are welding processes, wherein the welding wire gives deposited metal having sufficiently high hardness and improved antifrictional properties.